TM 357 weakness/leakage once every 3 shots

[Jimski]

Hello all,
as said in the title - I took apart my 357 to change some O-rings on the gas tank - after reconstruction, once every 3 or 4 shots, I get a dead fart instead of a push and the BB doesn't come out.

is there another o-ring inside around the percussion zone, that could be dead ?
could it be that the etancheity chain (from percutor system to barrel loading end) has a weak spot somewhere?

thanks if you have precisions or tricks.

I think i'm in for a full change of all O-rings ...